The representational structure of numbers in children's mental addition was investigated in this study. First- and fourth-grade children solved the addition problems in which the sum of addition was smaller than 10. The data on reaction times showed that the first-grade children solved the problems with 5 in either of the two addends faster than the problems without 5 in both addends. Furthermore, reaction times showed that the addition problems with the sum of 10 took less time to solve than the problems in which the sum was smaller than 10. These results suggested that children acquired representational structure of numbers to 5 as a privileged anchor. The results were discussed in terms of mental number line.